You probably can - although it's always best to check with the amp's maker.

As for the tubes you linked, they are not my personal favorite, even of VT-231's. I like the Sylvania VT-231 best, excluding the Tung-Sol black glass round plates, some of which were labeled VT-231 (the US military designation for a 6SN7). Those Ken-Rads will have a very robust bass, but most people I know find them a bit muddy, as do I.

I bought a 6SN7 to 12AU7 adapter off eBay. It works fine. There is a lot of discussion on this topic in this thread. Somewhere in the thread, Doc from Bottlehead chimes in and says the 6SN7 with an adapter works fine in the Crack amp.

I bought a 6SN7 to 12AU7 adapter off eBay. It works fine. There is a lot of discussion on this topic in this thread. Somewhere in the thread, Doc from Bottlehead chimes in and says the 6SN7 with an adapter works fine in the Crack amp.